MEMS includes integrated micro devices, such as mechanical, optical and thermal sensing components, formed on a substrate made of a single or composite layers of solid state materials. The MEMS is preferably fabricated by using the state-of-art wafer batch processing techniques to form those micro devices, sized from nanometers to millimeters, on a solid state substrate like a silicon wafer. Those MEMS devices are operating for sensing, controlling, and actuating various mechanical, optical or chemical functions on a micro scale, individually in single units or collaboratively in arrays for generating coordinated overall effects on a macro scale. Typical applications of such MEMS devices include, but not limited to, accelerometers, gyroscopes, pressure sensors, chemical and flow sensors, micro-optics devices, optical scanners, fluid flow control devices, chemical sensing and chemical delivery systems, and biological sensors among many others.
Furthermore, MEMS devices are preferably fabricated together in a unified process with supporting integrated circuit (IC) devices on the same semiconductor substrate as an integrated silicon device, namely as an integrated MEMS. Advantageously, such integrated MEMS in a single chip solution not only greatly reduce the size, weight and power consumption but also enhance the performance of an application system when compared with the conventional construction which separates MEMS and supporting IC as different micro devices.
Fabrication of MEMS devices employs many of the same processing steps as the fabrication of IC. In particular, the formation of an MEMS device involves depositing and patterning thin films on a substrate surface, such as a silicon wafer surface, to produce complex micro devices. Solid state thin film materials commonly used include but not limited to silicon dioxide, silicon nitride, polycrystalline silicon (poly), amorphous silicon, aluminum, copper, refractory metals and their oxide or nitride compounds.
However, to achieve certain mechanical, optical or thermal functions of MEMS devices, it is necessary to spatially decouple selected micro structural elements in MEMS devices to form a gap or cavity between the decoupled and the rest. Such decoupling of micro structural elements in MEMS devices enables certain desired mechanical, thermal, chemical or optical functions as required. For example, a number of MEMS motion sensors contain two or more micro structural elements which are spatially separated but could move relatively to each other. In many MEMS devices, cavities and suspended structural elements are necessities to be fabricated only through a wafer-level micro machining process. One of the most widely used approaches to form a gap or cavity between a top and bottom structural elements in an MEMS device involves selective etching a solid sacrificial layer or element. This sacrificial layer is first formed on the bottom structural element and then as the underline physical supporting base, enables deposition and patterning of the top structural element.
After depositing the sacrificial layer and forming the top structural element, photolithographic masking, patterning and etching steps are employed to remove the sacrificial layer, completely or at least partially. In general, such sacrificial etch processes fall into two categories, wet etching and dry etching. In a wet sacrificial etch process, the microstructure is immersed with the carrier wafer in or exposed to a liquid chemical bath containing an adequate etchant solution for dissolving and removing the sacrificial layer. This approach is very effective for forming a cavity or undercut on a large scale, ranging from tens or hundreds of micrometers. On a smaller scale, a number of drawbacks are encountered limiting a wet sacrificial etch process from extended application to micromachining of MEMS structural elements at an increased high precision and device density, preferably compatible to mainstream IC fabrication processes.
Overcoming many of those disadvantages and limitations, a dry etch process typically uses a gas as the primary etchant without accompanying wet chemicals or bath, less aggressive than wet processes, allowing the formation of smaller and more delicate structures on a substrate surface with the decreased risk of structure damage.
A number of sacrificial layer formation and according dry etch removal schemes with good etch selectivity over other materials are developed and applied to MEMS devices fabrication. The most well-known among all the reported schemes is using a developed photoresist as the sacrificial layer, depositing and patterning the top structural element and later, dry etch removing the underline partially exposed sacrificial photoresist layer via oxygen plasma ashing. The disadvantages of using photoresist as a sacrificial layer including poor mechanical support to its top structural element, low temperature tolerance to proceeding thin film processing, and out-gassing of residual chemical species after development among all. Other inorganic materials disclosed and used as a sacrificial layer in the prior art would resolve those issues with photoresist but most of them are exotic materials or its according dry etch removing process is not compatible to typical IC processing given the selectivity requirement to meet.